Conventionally, a crane including an upper turning body mounted on a lower body to turn freely has been known. The upper turning body of the crane includes a turning frame attached on the lower body to turn freely and a boom attached to the turning frame to be raised and lowered freely. Patent Literature 1 to 4 below disclose an example of such a crane.
Due to, for example, the influence of inertia force caused by turning of the upper turning body or of wind, force in the left-right direction is applied to the boom in some cases. In such cases, the boom deflects in the left-right direction. Particularly, in recent years, the amount of deflection of the boom in the left-right direction tends to increase, along with an increase in size of the crane or an increase in length of the boom. Under such circumstances, deflection of the boom in the left-right direction cannot be suppressed sufficiently with a conventional structure of the upper turning body, and work under limitations in hoisting load is increasing as a result.
In order to solve this problem, it is conceivable, for example, to increase the dimension (width) of the boom in the left-right direction and enhance the stiffness of the boom in the left-right direction. However, in this case, there occurs a problem that transportation of the turning frame of the upper turning body is difficult upon transportation of the crane. The reason is as follows.
As described in each patent literature below, a crane is in some cases transported after being disassembled into various equipment and members forming the crane. Therefore, a crane is configured to enable easy disassembly into units suitable for transportation.
An upper turning body of a conventional crane schematically includes a structure as shown in FIG. 20. The upper turning body includes a turning frame 720 and a boom 740. The front section of the turning frame 720 is provided with a pair of attachment brackets 721. The boom 740 includes a base end section attached to the pair of attachment brackets 721.
Upon transportation of the crane, the upper turning body is detached from a lower body. The detached upper turning body is disassembled into the turning frame 720, the boom 740, and various equipment, members, and the like mounted on the turning frame 720. Transportation of the crane is generally performed by a transportation vehicle such as a trailer. Upon the transportation, the turning frame 720 of the upper turning body is, in a state where the left-right direction thereof matches the left-right direction (car width direction) of the transportation vehicle and where the front-rear direction thereof matches the front-rear direction (car length direction) of the transportation vehicle, loaded on the transportation vehicle. Regarding vehicles passing through public roads, a limit value of the dimension in the left-right direction including a loaded object, i.e., transportation limit width, is specified by laws and regulations. Therefore, it is desired that the dimension in the left-right direction be kept less than or equal to the transportation limit width also for components of the crane, in a state of being loaded on the transportation vehicle.
However, when a dimension WB of the boom 740 in the left-right direction is increased in order to suppress deflection of the boom 740 in the left-right direction, it is necessary to increase the interval between the pair of attachment brackets 721 to which the base end section of the boom 740 is attached. In this case, a dimension (width) WF of the turning frame 720 in the left-right direction also has to be increased. As a result, when the turning frame 720 is to be loaded on a transportation vehicle and transported as described above, the dimension WF of the turning frame 720 in the left-right direction exceeds the transportation limit width in some cases. In such cases, there is a risk that the turning frame 720 cannot be transported.